Cancer is one of the leading causes of death in the developed world, with over one million people diagnosed with cancer and 500,000 deaths per year in the United States alone. Overall it is estimated that more than 1 in 3 people will develop some form of cancer during their lifetime. There are more than 200 different types of cancer, four of which—breast, lung, colorectal, and prostate—account for over half of all new cases (Jemal et al., 2003, Cancer J. Clin. 53:5-26).
The Wnt signaling pathway has been identified as a potential target for cancer therapy. The Wnt signaling pathway is one of several critical regulators of embryonic pattern formation, post-embryonic tissue maintenance, and stem cell biology. More specifically, Wnt signaling plays an important role in the generation of cell polarity and cell fate specification including self-renewal by stem cell populations. Unregulated activation of the Wnt pathway is associated with numerous human cancers where it can alter the developmental fate of tumor cells to maintain them in an undifferentiated and proliferative state. Thus carcinogenesis can proceed by usurping homeostatic mechanisms controlling normal development and tissue repair by stem cells (reviewed in Reya & Clevers, 2005, Nature 434:843; Beachy et al., 2004, Nature 432:324).
The Wnt signaling pathway was first elucidated in the Drosophila developmental mutant wingless (wg) and from the murine proto-oncogene int-1, now Wnt1 (Nusse & Varmus, 1982, Cell 31:99-109; Van Ooyen & Nusse, 1984, Cell 39:233-40; Cabrera et al., 1987, Cell 50:659-63; Rijsewijk et al., 1987, Cell 50:649-57). Wnt genes encode secreted lipid-modified glycoproteins of which 19 have been identified in mammals. These secreted ligands activate a receptor complex consisting of a Frizzled (Fzd) receptor family member and low-density lipoprotein (LDL) receptor-related protein 5 or 6 (LPR5/6). The Fzd receptors are seven transmembrane domain proteins of the G-protein coupled receptor (GPCR) superfamily and contain a large extracellular N-terminal ligand binding domain with 10 conserved cysteines, known as a cysteine-rich domain (CRD) or Fri domain. There are ten human FZD receptors: FZD1-10. Different Fzd CRDs have different binding affinities for specific Wnts (Wu & Nusse, 2002, J. Biol. Chem. 277:41762-9), and Fzd receptors have been grouped into those that activate the canonical β-catenin pathway and those that activate non-canonical pathways described below (Miller et al., 1999, Oncogene 18:7860-72). To form the receptor complex that binds the FZD ligands, FZD receptors interact with LRP5/6, single pass transmembrane proteins with four extracellular EGF-like domains separated by six YWTD amino acid repeats (Johnson et al., 2004, J. Bone Mineral Res. 19:1749).
The canonical Wnt signaling pathway activated upon receptor binding is mediated by the cytoplasmic protein Dishevelled (Dsh) interacting directly with the Fzd receptor and results in the cytoplasmic stabilization and accumulation of β-catenin. In the absence of a Wnt signal, β-catenin is localized to a cytoplasmic destruction complex that includes the tumor suppressor proteins adenomatous polyposis coli (APC) and Axin. These proteins function as critical scaffolds to allow glycogen synthase kinase (GSK)-3β to bind and phosphorylate β-catenin, marking it for degradation via the ubiquitin/proteasome pathway. Activation of Dsh results in phosphorylation of GSK3β and the dissociation of the destruction complex. Accumulated cytoplasmic β-catenin is then transported into the nucleus where it interacts with the DNA-binding proteins of the Tcf/Lef family to activate transcription.
In addition to the canonical signaling pathway, Wnt ligands also activate β-catenin-independent pathways (Veeman et al., 2003, Dev. Cell 5:367-77). Non-canonical Wnt signaling has been implicated in numerous processes but most convincingly in gastrulation movements via a mechanism similar to the Drosophila planar cell polarity (PCP) pathway. Other potential mechanisms of non-canonical Wnt signaling include calcium flux, JNK, and both small and heterotrimeric G-proteins. Antagonism is often observed between the canonical and non-canonical pathways, and some evidence indicates that non-canonical signaling can suppress cancer formation (Olson & Gibo, 1998, Exp. Cell Res. 241:134; Topol et al., 2003, J. Cell Biol. 162:899-908). Thus, in certain contexts, Fzd receptors act as negative regulators of the canonical Wnt signaling pathway. For example, FZD6 represses Wnt-3a-induced canonical signaling when co-expressed with FZD1 via the TAK1-NLK pathway (Golan et al., 2004, JBC 279:14879-88). Similarly, Fzd2 antagonized canonical Wnt signaling in the presence of Wnt-5a via the TAK1-NLK MAPK cascade (Ishitani et al., 2003, Mol. Cell. Biol. 23:131-9).
The canonical Wnt signaling pathway also plays a central role in the maintenance of stem cell populations in the small intestine and colon, and the inappropriate activation of this pathway plays a prominent role in colorectal cancers (Reya & Clevers, 2005, Nature 434:843). The absorptive epithelium of the intestines is arranged into villi and crypts. Stem cells reside in the crypts and slowly divide to produce rapidly proliferating cells that give rise to all the differentiated cell populations that move up out of the crypts to occupy the intestinal villi. The Wnt signaling cascade plays a dominant role in controlling cell fates along the crypt-villi axis and is essential for the maintenance of the stem cell population. Disruption of Wnt signaling either by genetic loss of Tcf7/2 by homologous recombination (Korinek et al., 1998, Nat. Genet. 19:379) or overexpression of Dickkopf-1 (Dkk1), a potent secreted Wnt antagonist (Pinto et al., 2003, Genes Dev. 17:1709-13; Kuhnert et al., 2004, Proc. Nat'l. Acad. Sci. 101:266-71), results in depletion of intestinal stem cell populations.
Colorectal cancer is most commonly initiated by activating mutations in the Wnt signaling cascade. Approximately 5-10% of all colorectal cancers are hereditary with one of the main forms being familial adenomatous polyposis (FAP), an autosomal dominant disease in which about 80% of affected individuals contain a germline mutation in the adenomatous polyposis coli (APC) gene. Mutations have also been identified in other Wnt pathway components including Axin and β-catenin. Individual adenomas are clonal outgrowths of epithelial cell containing a second inactivated allele, and the large number of FAP adenomas inevitably results in the development of adenocarcinomas through addition mutations in oncogenes and/or tumor suppressor genes. Furthermore, activation of the Wnt signaling pathway, including gain-of-function mutations in APC and β-catenin, can induce hyperplastic development and tumor growth in mouse models (Oshima et al., 1997, Cancer Res. 57:1644-9; Harada et al., 1999, EMBO J. 18:5931-42).
A role for Wnt signaling in cancer was first uncovered with the identification of Wnt1 (originally int1) as an oncogene in mammary tumors transformed by the nearby insertion of a murine virus (Nusse & Varmus, 1982, Cell 31:99-109). Additional evidence for the role of Wnt signaling in breast cancer has since accumulated. For instance, transgenic overexpression of β-catenin in the mammary glands results in hyperplasias and adenocarcinomas (Imbert et al., 2001, J. Cell Biol. 153:555-68; Michaelson & Leder, 2001, Oncogene 20:5093-9) whereas loss of Wnt signaling disrupts normal mammary gland development (Tepera et al., 2003, J. Cell Sci. 116:1137-49; Hatsell et al., 2003, J. Mammary Gland Biol. Neoplasia 8:145-58). More recently mammary stem cells have been shown to be activated by Wnt signaling (Liu et al., 2004, Proc. Nat'l Acad. Sci. 101:4158). In human breast cancer, β-catenin accumulation implicates activated Wnt signaling in over 50% of carcinomas, and though specific mutations have not been identified, upregulation of Frizzled receptor expression has been observed (Brennan & Brown, 2004, J. Mammary Gland Neoplasia 9:119-31; Malovanovic et al., 2004, Int. J. Oncol. 25:1337-42).
FZD10, FZD8, FZD7, FZD4, and FZD5 are five of ten identified human Wnt receptors. Fzd10 is co-expressed with Wnt7b in the lungs, and cell transfection studies have demonstrated that the Fzd10/LRP5 co-receptor activates the canonical Wnt signaling pathway in response to Wnt7b (Wang et al., 2005, Mol. Cell Biol. 25:5022-30). FZD10 mRNA is upregulated in numerous cancer cell lines, including cervical, gastric, and glioblastoma cell lines, and in primary cancers including approximately 40% of primary gastric cancers, colon cancers, and synovial sarcomas (Saitoh et al., 2002, Int. J. Oncol. 20:117-20; Terasaki et al., 2002, Int. J. Mol. Med. 9:107-12; Nagayama et al., 2005, Oncogene 1-12). FZD8 is upregulated in several human cancer cell lines, primary gastric cancers, and renal carcinomas (Saitoh et al., 2001, Int. J. Oncol. 18:991-96; Kirikoshi et al., 2001, Int. J. Oncol. 19:111-5; Janssens et al., 2004, Tumor Biol. 25:161-71). FZD7 is expressed throughout the gastrointestinal tract and is up-regulated in one out of six cases of human primary gastric cancer (Kirikoshi et al., 2001, Int. J. Oncol. 19:111-5). Expression of the FZD7 ectodomain by a colon cancer cell line induced morphological changes and decreased tumor growth in a xenograft model (Vincan et al., 2005, Differentiation 73:142-53). FZD5 plays an essential role in yolk sac and placental angiogenesis (Ishikawa et al., 2001, Dev. 128:25-33) and is upregulated in renal carcinomas in association with activation of Wnt/β-catenin signaling (Janssens et al., 2004, Tumor Biology 25:161-71). FZD4 is highly expressed in intestinal crypt epithelial cells and is one of several factors that display differential expression in normal versus neoplastic tissue (Gregorieff et al., 2005, Gastroenterology 129:626-38). The identification of FZD receptors as markers of cancer stem cells thus makes these proteins ideal targets for cancer therapeutics.